The present invention relates to an optical apparatus such as a camera, and more specifically, to focusing operation and control thereof.
In recent years, so-called digital cameras have been prevalent in which an image-taking optical system focuses a subject image onto a semiconductor image-pickup device (such as a CCD sensor and a CMOS sensor) for conversion into an electric signal and the resulting image information is recorded on a recording medium such as a semiconductor memory and a magnetic disk.
The electronic camera of this type is equipped with an auto-focus (AF) function of controlling image-taking conditions in order to automatically focus on a subject image. As methods of the auto-focus control, a contrast AF method and a phase difference detection AF method are adopted.
Known contrast AF methods include a climbing method in which a focus lens is moved in a direction in which the high-frequency component of a luminance signal (hereinafter referred to as an AF evaluation value (sharpness)) provided by an image pick-up device increases and the position of the lens where the AF evaluation value is at the maximum is set to an in-focus position, and a full range scan method in which AF evaluation values are stored while a focus lens is driven throughout the distance-measuring range and the position of the lens corresponding to the maximum of the stored AF evaluation values is set to an in-focus position. In particular, the contrast AF method is widely utilized since the image-pickup device for imaging is used without any change and the AF evaluation value is detected from output from the image-pickup device.
Single-lens reflex cameras employ the phase difference detection AF method. In the phase difference detection method, luminous fluxes are passed through the mirror areas of a plurality of different image-taking lenses for an imaging plane and an optical system is provided for secondary image formation of the luminous fluxes.
The two images formed through the secondary image formation are detected by two line sensors, and the phase difference between the data of the two images is detected to determine the defocus state (amount) of the subject image. The in-focus position is calculated and necessary lens driving is performed, thereby realizing control to achieve an in-focus state.
In addition, focus control of a hybrid AF method has been proposed as a combination of these AF methods. In the hybrid AF method, a focus lens is driven to near an in-focus point in the phase difference detection AF method and then the focus lens is drive to the in-focus position more accurately in the contrast AF method, by way of example (Japanese Patent Laid-Open No. 7-043605).
The focus detection in the contrast AF method, however, can only cover a narrow range of defocus amounts, so that the focus detection is difficult to perform when the image is significantly out of focus. In addition, since it takes a long time to move the focusing lens from the infinity end to the closest end in order to search for an in-focus position, the method is inappropriate for a system requiring fast operation and image-taking of a subject who moves quickly. Moreover, the high-frequency component of a luminance signal is not changed largely at positions away from an in-focus position, so that it is difficult to know whether defocus is caused by front focus or rear focus.
The focus detection in the phase difference detection AF method has the problem of involving a dead zone in its focus detection area although it can cover a wide range of defocus amounts. Also, since the movement amount of a focus lens is determined on the basis of a difference between the positions of the image formation on the image-pickup device, the AF range is limited in some of image-pickup devices and lens systems. Thus, when the infinite end and the closest end are greatly separated from each other and a lens system is set to allow AF control throughout the image-taking range, resolution may be reduced by reason of the size of the image-pickup device or the like to result in reduced AF accuracy.
In view of those situations, the hybrid AF method disclosed in Japanese Patent Laid-Open No. 7-043605 attempts to address the abovementioned problems by performing coarse adjustment with the focus detection of the phase difference detection method and fine adjustment with the focus detection of the contrast method, and then focusing operation of the focus lens.
However, the focus detection of the contrast method is performed at all times even when only the focus detection of the phase difference detection method can provide sufficient auto-focus in terms of focus accuracy, so that focusing operation takes a long time. This may cause missing of an opportunity to take images of a quickly moving subject or require a long time for each image-taking operation.